Fig. 1 - Forests in transition areas next to meadows, pasture land, moor land and dry sites are rich in rare plants.
Photo: Thomas Stalling
Direct surveys of different plant species are usually adequate instruments for recording plant species biodiversity. Scientists have however examined the possibility of whether the species diversity of vascular plants, mosses and molluscs could also be indirectly recorded using forest structure or habitat characteristics. The answer is: only partially.
Habitat surveys are usually recommended for species diversity observation, rather than direct species surveys, because they are cheaper. Using data from the Swiss National Forest Inventory (NFI) and the Swiss Biodiversity Monitoring Programme (BDM) scientists from the Federal Research Institute WSL examined to what extent vascular plant, moss and mollusc density could be linked to 58 different forest characteristics including structure, forest use and site.
The BDM monitoring network for species within their habitat is located on a sub network of the NFI. This means that data about the occurrence of vascular plants, mosses and molluscs is available from the BDM for part of the NFI-sample plots. Therefore, forest characteristics (data from NFI) can be compared to species diversity (data from BDM) for the same area within the forest stand.
From the data catalogue of the second NFI and the BDM scientists chose a total of 58 site and structural characteristics which could play a significant role in forest biodiversity. In the survey species density was used as an indicator for species diversity, that is to say, the amount of species per group for each BDM test area.
For the data analysis the scientists used the so called "linear regression" model to quantify a relationship between one or more forest characteristics and species diversity. They searched for a combination of characteristics with which differences between sample plots in regard to species diversity could best be explained. The search for a model was carried out in three stages: the first stage only took into account site factors, the second stage structural characteristics and in the third stage the best model from the first stage was supplemented with characteristics from the second stage.
Structural characteristics, which can be influenced by cultivation, account for another 5% (molluscs), 7% (mosses) and 15% (vascular plants) of the recorded differences. Therefore the best overall models explain 33% (vascular plants), 32% (mosses) and 54% (molluscs) of the found differences in species diversity per 10 m² of soil area.
Due to the variety of forest types it was expected that site factors would explain a large part of the species diversity within the sample plots. In the case of mosses, and especially in the case of molluscs, site conditions do actually play a more important role than forest structure (Fig.2).
Fig. 2 - Old trees with thick gnarled bark provide optimal growing conditions for mosses.
Photo: Norbert Schnyder
Out of all modifiable structural characteristics in a forest stand only lighting conditions play an important role especially for vascular plants. There seems to be a trend which shows that light open forests generally have a higher species diversity than dark dense forests.
It has commonly been assumed that the heterogeneity of a forest stand (age, structure, vertical layering or the degree of mixture) would encourage biodiversity. However this could be observed only very slightly in the case of molluscs. This is probably due to the fact that, as opposed to small mammals and birds, molluscs, mosses and vascular plants very rarely use the vertical dimension of a stand.
On average forests with the greatest plant diversity harbour the largest number of rare forest plants, which have their main distribution outside of forests. These are meadow, bog or ruderal plants. The less dense the stocking the greater the percentage of these plants. Therefore, forests which are most rich in species are not forests with typical closed stands, but those which are rich in herbaceous plants, that is to say forests in transition areas next to meadows, pasture land, marshes and dry areas. (Fig.1).
The results of the above analysis show that between 32 and 54% of the diversity of vascular plants, mosses and molluscs within the 10 m² BDM sample plots can be explained, depending on the species, by using site and structural characteristics from the NFI and BDM. Even though all relevant habitat factors have probably not been included and the size of the sample plots of the NFI and BDM differ by a factor of 250 the relationship is very close. However, the examined areas are not sufficient to deliver reliable conclusions about the development of small scale plant diversity. For such purposes direct field surveys are indispensible.
If results cannot however be verified by species surveys then the authors suggest the following indicators be used within the framework of forest observation:
Numerous other studies confirm the above described relationship and show the important ecological role habitat factors (structural characteristics) play in species diversity, especially in the case of fungi, insects, birds and small mammals. That is why experts recommend that further forest characteristics, for instance deadwood, be recorded.
The general aim of biodiversity preservation and promotion is not a maximisation of the number of species within a small area. However, a suitable goal could be an increase in plant diversity in widespread monotone commercial forests. The study shows that the following recommendations for the researched plant groups (vascular plants, mosses, molluscs) could lead to the desired results:
Improved light conditions encourage plant diversity more than any other factor. This is especially important for vascular plants, where an increase in light brings an increase in the percentage of untypical forest plants (forest and forest edge as a refuge). Thinning should be carried out without removing deadwood and snags from the forest. Deadwood should be left and piled into heaps.
Deciduous and mixed forests house the most mollusc species especially in damp cool sites and also in alluvial forests.